Abstract The transforming growth factor β (TGFβ) signaling pathway is a pleiotropic cellular pathway that plays a critical role in cancer. In fact, aggressive tumors are typically associated with high ligand levels and thus associated with poor prognosis in various tumor types. Cancer cells use autocrine and paracrine TGFβ signaling to modulate tumor cells and the tumor microenvironment leading to a highly invasive and metastatic phenotype, inducing and increasing tumor vascularization, modulating the extracellular matrix in the stroma, and inhibiting immune surveillance and antitumor immunity. Clinical studies with galunisertib (aka LY2157299 monohydrate), a small molecule inhibitor targeting the TGFβ pathway, have provided proof of concept data supporting the role of TGFβ in cancer and the utility of targeting the TGFβ pathway. Here we describe the identification of LY3200882, a next generation small molecule inhibitor of TGF-β receptor type 1 (TGFβRI). The molecule is a potent, highly selective inhibitor of TGFβRI embodied in a structural platform with a synthetically scalable route. It is an ATP competitive inhibitor of the serine-threonine kinase domain of TGFβRI. Mechanism of action studies reveal revealed that LY3200882 inhibits various pro-tumorigenic activities. LY3200882 potently inhibits TGFβ mediated SMAD phosphorylation in vitro in tumor and immune cells and in vivo in subcutaneous tumors in a dose dependent fashion. In preclinical tumor models, LY3200882 showed potent anti-tumor activity in the orthotopic 4T1-LP model of triple negative breast cancer and this activity correlated with enhanced tumor infiltrating lymphocytes in the tumor microenvironment. Durable tumor regressions in the orthotopic 4T1-LP model were observed and rechallenge of congenic tumors resulted in complete rejection in all mice. In in vitro immune suppression assays, LY3200882 has shown the ability to rescue TGFβ1 suppressed or T regulatory cell suppressed naïve T cell activity and restore proliferation. Therefore, LY3200882 shows promising activity as an immune modulatory agent. In addition, LY3200882 has shown anti-metastatic activity in vitro in migration assays as well as in vivo in an experimental metastasis tumor model (intravenous EMT6-LM2 model of triple negative breast cancer). Finally, LY3200882 shows combinatorial anti-tumor benefits with checkpoint inhibition (anti-PD-L1) in the syngeneic CT26 model. In conclusion, we have developed a novel potent and highly selective small molecule inhibitor of TGFβRI for the treatment of cancer. Citation Format: Huaxing Pei, Saravanan Parthasarathy, Sajan Joseph, William McMillen, Xiaohong Xu, Stephen Castaneda, Ivan Inigo, Karen Britt, Bryan Anderson, Gaiying Zhao, Scott Sawyer, Douglas Beight, Talbi Kaoudi, Chandrasekar Iyer, Huimin Bian, Amy Pappas, David Surguladze, David Schaer, Karim Benhadji, Michael Kalos, Kyla Driscoll. LY3200882, a novel, highly selective TGFβRI small molecule inhibitor [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 955. doi:10.1158/1538-7445.AM2017-955